Technology and methodology has been developed to perform micro spectrophotofluorometry in caged laboratory animals using miniature fiber optic probes. The technique is being used initially for microwashout studies of tumors in rats using fluorescein injected intravenously to investigate variations in tissue perfusion in the tumors. In the next year, studies of tumor capillary permeability will be performed; measurements made with dextrans of varying molecular weight and labeled with fluorescein will be combined with analytical modeling. In the future, we hope to employ the technique to study metabolic modeling. In the future, we hope to employ the technique to study metabolic behavior of tumors and pharmacokinetics of suitably fluorescent drugs in animals.